Supporting Online Material for

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1 Supporting Online Material for Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction Kuanping Gong, Feng Du, Zhenhai Xia, Michael Durstock, Liming Dai* This PDF file includes: *To whom correspondence should be addressed. Materials and Methods Figs. S1 to S6 Table S1 References Published 6 February 2009, Science 323, 760 (2009) DOI: /science

2 SUPPORTING ONLINE INFORMATION Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction Kuanping Gong 1, Feng Du 1, Zhenhai Xia 2, Michael Durstock 3 & Liming Dai 1,* 1 Departments of Chemical and Materials Engineering and Chemistry and UDRI, University of Dayton, 300 College Park, Dayton, OH 45469, USA 2 Department of Mechanical Engineering, University of Akron, Akron, OH44325, USA 3 Materials and Manufacturing Directorate, Air Force Research Laboratory, AFRL/RXBP, Wright-Patterson AFB, OH 45433, USA *Correspondence should be addressed to L.D. (ldai@udayton.edu) 1. Materials and Characterization Materials. Vertically-aligned nitrogen-containing carbon nanotube (VA-NCNT) arrays were prepared by pyrolyzing iron(ii) phthalocyanine (FePc) at C on a quartz glass plate according to the published method in the absence and presence of additional NH3 gas (S1, S2). In the latter case, mixture gases of 48% Ar, 28% H 2, and 24% NH3 were flown through the quartz tube furnace. Vertically-aligned nitrogen-free carbon nanotubes were prepared by following the similar procedures using ferrocene as the precursor molecule. The commercially available carbon nanotubes (denoted as: CCNT) with an average diameter of nm were purchased from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China) and used as reference after purification through the reported procedure (S3). Non-aligned NCNTs (NA-NCNTs) were obtained by sonicating the as-synthesized VA-NCNTs in double-distilled water for 1 hour and collected through centrifugation, followed by drying in vacuum oven (150 C) prior to subsequent analyses. Platinum loaded carbon electrode (Pt-C, C2-20, 20% HP Pt on Vulcan 1

3 XC-72R) was purchased from E-TEK Division, PEMEAS Fuel Cell Technologies. All other chemicals were purchased from Sigma-Aldrich and used without any further purification. Electrode preparation. Fig. S1A shows a schematic representation of the procedures used for the electrode preparation. As can be seen, a thin layer (~1 μm) of polystyrene (PS) and nonaligned NCNTs (NA-NCNTs) conductive composite film was coated from its solution (10 wt% PS and 2.0 mg/ml NCNTs in toulene) on the top surface of the as-synthesized VA-NCNT array (Step 1 of Fig. S1A), followed by heating up to 185 o C in air for about 1 minute to ensure a strong adhesion to the VA-NCNT array through a controlled slight infiltration of the melted PS and NA-NCNT composite into the VA-NCNT array (Step 2 of Fig. S1A) (S4). Thereafter, H 2 O-plasma etching (S5) was applied to the free surface of the PS-NCNT coating to expose carbon nanotubes (Step 3 of Fig. S1A). The quartz substrate was then removed by immersing it in an aqueous HF solution (1:6 v/v) (S1) to produce a free-standing VA-NCNT film supported by the conductive PS and NA-NCNT coating (Fig. 1C, Step 4 of Fig. S1A). Finally, the resultant free-standing VA-NCNT film was attached onto a glassy carbon (GC) electrode to fully cover the 3- or 5-mm electroactive circle [Fig. S1B (Bioanalytical Systems Inc.) and Fig S1C (Pine Instrument Co.)] with the NA-NCNT exposed surface on the PS and NA-NCNT composite binder facing down onto the GC electrode to ensure an intimate conductive connection between the VA-NCNT array and the underlying electroactive core of the GC electrodes. Prior to subsequent electrochemical measurements in electrochemical cells with the gas inlet and outlet for introducing the protection gas (Figs. S1B and S1C), the VA-NCNT/GC electrode thus prepared was purified by electrochemical oxidation in a phosphate buffered solution (ph 6.8) at a potential of 1.7 V (vs. Ag/AgCl) for 300 s at room temperature (25 ± 2

4 1 C), followed by potential sweeping from 0.0 V to 1.4 V in 0.5 M H 2 SO 4 until a stable voltommagram was achieved. As reference, glassy-carbon-supported nonaligned nitrogen-containing carbon nanotube (NA-NCNT/GC), nonaligned nitrogen-free carbon nanotube (NA-CCNT/GC), and vertically-aligned nitrogen-free carbon nanotube (VA-CCNT/GC) electrodes were also prepared following the same procedure using the PS binders containing the corresponding nanotubes as the conductive component and subjected to the same electrochemical purification procedures to remove residual Fe catalyst, if any. (A) (B) (C) Fig. S1. (A) Schematic illustration of the procedures for the electrode preparation. (B) Electrochemical cells with the gas inlet and outlet and electrodes for normal cyclic voltammogram (a) and RRDE voltammogram (b) measurements. 3

5 Characterization. Electrochemical measurements were performed using a computer-controlled potentiostat (CHI 760C, CH Instrument, USA) with a typical three-electrode cell equipped with gas flow systems (Figs. S1B and S1C). VA-NCNT/GC, NA-NCNT/GC, VA-CCNT/GC, NA-CCNT/GC, or Pt-C/GC electrode was used as working electrode, an Ag/AgCl (3 M KCl-filled) electrode as reference electrode, and a platinum wire as counter electrode. An aqueous solution of KOH (0.1 M) was used as electrolyte for both normal cyclic voltammogram and rotating ring-disk electrode (RRDE) voltammogram measurements. Normal cyclic voltammograms were collected with a modified GC disk electrode, which is composed of a glassy carbon core of 3 mm in diameter and a surrounding insulation area of 6 mm in diameter (Fig. S1B), while RRDE voltammograms were recorded using a modified GC ring-disk electrode with a Pt ring on a 5-mm diameter glassy carbon core and 9-mm outer diameter (Fig. S1C). The collection efficiency of the rotating ring-disk electrode was determined to be 0.30 with Fe(CN) 4-/3-6 probe. RRDE experiments were carried out on a MSRX speed controller (Pine Instrument Co.) and the CHI 760C bipotentiostat. Potential-sweep electrolysis was performed at the modified GC disk electrode while the Pt ring electrode was polarized at 0.50 V for oxidizing HO 2 - intermediate, if any, from the disk electrode. All the experiments were conducted at room temperature (25 ± 1 C). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were recorded on a Hitachi S4800-F high-resolution SEM and H-7600 TEM unit (Hitachi, Japan), respectively. X-ray photoelectron spectroscopic (XPS) measurements were performed on a VG Microtech ESCA 2000 using a monochromic Al X-ray source. Thermogravimetric analyses were performed on a TGA Q500 unit (TA). 4

6 XPS: XPS survey and high-resolution C1s spectra for the VA-NCNTs before and after the electrochemical purification are given in Fig. S2. As expected for the as-grown VA-NCNT array (Fig. S2A), the XPS survey spectrum shows a predominant C 1s peak at 285 ev and a much weaker O 1s peak at 531 ev, presumably arising from physically adsorbed oxygen-containing species (S6). The Fe signals at 55 and 707 ev were attributable to the residual catalyst in the FePc-generated VA-NCNTs. As can be seen in the inset of Fig. S2A, the peak at 399 and 401 ev can be attributed to pyridinic-like and pyrrolic-like groups, respectively, indicating the incorporation of nitrogen within the graphene sheets (S7). Initially, the N atoms could preferentially incorporate into the hexagon graphene, as reflected by the pyridinic-like XPS peak at 399 ev. The continuous presence of N atoms in the furnace could introduce N-containing pentagon defects inside the hexagon network (S8). The corresponding XPS survey spectrum for the electrochemically purified VA-NCNTs array (Fig. S2C) shows similar peaks as Fig. S2A, but with a much stronger O 1s peak and an almost negligible intensity for the Fe signals. These features are consistent with the removal of residual iron catalyst particles and the generation of certain oxygen-containing surface groups by electrochemical purification. The high-resolution C1s spectrum of the as-grown VA-NCNT array (Fig. S2B) shows a relatively narrow graphitic C at about 285 ev (S9), whereas the corresponding high-resolution C1s spectrum for the electrochemically purified VA-NCNT array given in Fig. S2D shows three peak components at 284.6, and ev, which are assigned to the carbon component in C-C / C-H, C-O and O=C-O, respectively. The XPS survey and high-resolution C1s for the VA-NCNT array after the oxygen reduction reaction (ORR) (Figs. S2E and S2F) show similar features as those shown in Figs. S2C and S2d, respectively. The absence of any 5

7 obvious change in the XPS C 1s spectra for the electrochemically purified NCNT before and after the ORR process, coupled with the lack of a 2e-pathway for the ORR associated with oxidized carbon nanotubes (Fig. 2), indicate that oxygen-containing surface groups in the electrochemically-purified NCNTs are inactive (S10) toward the ORR in the present study, though they could facilitate O 2 adsorption to a certain extent. As seen in Table S1, there is considerable nitrogen content (ca.4 to 6 at.%) in all of the N-containing carbon nanotubes used in this study. Our measurements showed that the ORR electroactivities increased with increasing the nitrogen content within the range covered by this study. Thus, the XPS results indicate that the high ORR catalytic activity of the NCNTs essentially originated from the active sites of pyridinic /pryrrolic nitrogens in consistence with the quantum mechanics calculations (vide infra). Table S1. N/C atomic ratios for the N-containing carbon nanotubes from the XPS data (±0.5%) C (at. %) N (at. %) N : C (at. %) Raw VA-NCNT (FePc) Purified VA-NCNT (FePc, before ORR) Purified VA-NCNT (FePc, after ORR) Raw VA-NCNT (FePc / NH 3 )

8 A B C D E F Fig. S2. XPS survey and high-resolution C1s for the as-synthesized VA-NCNTs (A, B), and the electrochemically purified VA-NCNTs before (C, D) and after the ORR measurement (E, F). Inset of (A) shows the N 1s spectra for the as-synthesized VA-NCNTs. TGA: To start with, we mechanically scratched the as-synthesized VA-NCNTs from several pieces of the quartz plates to gain enough materials for the thermogravimetric analysis (TGA, TA500). The dot curve in Fig. S3 reveals approximately 12% (by weight) orange-colored residue even after being heated up to ca C due, most probably, to the presence of Fe catalyst residues that became Fe 2 O 3 upon the thermal oxidation. 7

9 Fig. S3. Thermogravimetric data of VA-NCNT sample in O 2 before and after the electrochemical purification. Scan rate 20 o C/min To perform the corresponding TGA measurement for the electrochemically purified N-containing nanotubes, we first carried out the electrochemical oxidation of the nonaligned NCNTs (mechanically scratched from several VA-NCNT arrays) in a phosphate buffered solution (ph 6.8) at a potential of 1.7 V (vs. Ag/AgCl) for 300 s, followed by potential sweeping from 0.0 V to 1.4 V in 0.5 M H 2 SO 4 until a stable voltommagram was achieved, as mentioned earlier. We then washed the electrochemically purified NCNTs repeatedly with an excess of pure water, followed by vacuum drying overnight. As seen in Fig. S3 (solid curve), the purified NCNT sample showed almost no residue above 550 C, indicating that the electrochemical purification has completely removed Fe catalyst residues in a good consistence with the XPS measurements (Fig. S2). Crossover effect. As can be seen in Fig. S4A, a significant loss in the open-circuit potential (ca. 110 mv) and steady-state output potential (150 mv), along with a reduction of 0.4 ma cm -2 in the maximum current density output (S11, S12), was observed for the Pt-C/GC electrode in 8

10 air-saturated 0.1 M KOH after adding 3.0 M methanol. In contrast, the corresponding effect on the VA-NCNT/GC electrode is almost negligible (Fig. S4B). A B Fig. S4. Half-cell I-V polarization curves for O 2 reduction at the Pt-C/GC (A) and VA-NCNT/GC (B) electrodes in air-saturated 0.1 M KOH in the absence (solid circles) and presence (open circles) of 3.0 M methanol. 2. Quantum Mechanics Calculations Pure and nitrogen-doped single-walled carbon nanotubes (CNT) was studied using a cluster approach with the B3LYP hybrid density functional theory (DFT) in Gaussian 03 (S13, S14). The basis sets used for N, and H were 6-31G**. Models used in this work are (5,5) armchair nanotubes with delocalized σ electrons and terminated with C-H bonds. Three types of nanotubes are considered. The first type is an ideal (pure) nanotube with a length of 12.5 Å (Fig. S5A); the second one is identical to the first one but N-doped CNT (NCNT), in which a nitrogen atom was placed substitutionally in the middle of the nanotube as shown in Fig. S5B. The last type contains the same number of C and N atoms as the second one but with three merged Stone-Wales defects (NCNT5577) (Fig. S5C), where one C atom on one of the pentagon rings is replaced by the N atom. 9

11 (A) N (B) N (C) Fig. S5. Schematics of H-terminated carbon nanotubes: (A) ideal structure, (B) NCNT and (C) NCNT structure with three merged Stone-Wales defects. Fig. S6 shows charge densities of those three types of nanotubes. For the ideal carbon nanotube, the carbon atoms near its H-terminated end shows positive and negative charge due to hydrogen bonding (Fig. S6A) but the positive charge is very weak (~0.06). In this case, carbon atoms a few hexagon away from the H-terminated end become neutral (S14). For defect-free N-doped nanotubes (Fig. S6B), the N atom is quite negative, with a charge of This is a consequence of N being a radical and more electronegative than carbon. Most of the compensating positive charge is distributed on the three neighboring carbon atoms with a 10

12 positive charge of 0.1. For the N-doped nanotube with the defects (Fig. S6C), it is interesting that a much higher positive charge of ~0.25 is found on a C atom in a pentagon ring adjacent to the N atom (Fig. S6C). The optimized geometry of the N-doped nanotube is as nearly perfect as the ideal nanotube but it slightly convexes near the N atom. H N N (A) (B) (C) Fig. S6. Charge distributions for carbon nanotube in its optimized structures. (A) ideal CNT near its H-terminated end, (B) NCNT without the pentagon defect and (C) NCNT with three Stone-Wales defects (5577 defects). References S1. S. M. Huang, L. Dai, A. W. H. Mau, J. Phys. Chem. B, 103, 4223 (1999). S2. L. S. Panchakarla, A. Govindaraj, C. N. R. Rao, ACS Nano 1, 494 (2007). S3. K. Gong, P. Yu, L. Su, S. Xiong, L. Mao, J. Phys. Chem. C 111, 1882 (2007). S4. L. Qu, L. Dai, Chem. Commun, 3859 (2007). S5. L. Dai, H. J. Griesser, A. W. H. Mau, J. Phys. Chem. B 101, 9548 (1997). S6. Q. Chen, L. Dai, M. Gao, S. Huang, A. W. H. Mau, J. Phys. Chem. B 105, 618 (2001). S7. S. Maldonado, K. J. Stevenson, J. Phys. Chem. B 108, (2004). S8. C. Morant et al., Phys. Stat. Sol. A, 203, 1069 (2006). S9. M. Yudasaka, R. Kikuchi, Y. Ohki, S. Yoshiura, Carbon 35, 195 (1997). S10. K. Gong, S. Chakrabarti, L. Dai, Angew. Chem. Int. Ed. 47, 5446 (2008). 11

13 S11. S. Basu, Ed., Recent Trends in Fuel Cell Science and Technology (Springer, New York, 2007). S12. A. J. Appleby, J. Electroanal. Chem. 357, 117 (1993). S13. M. J. Fisch et al., Gaussian 03, Revision B.04 (Gaussian, Inc., Pittsburgh, PA, 2003). S14. R. A. Sidik et al., J. Phys. Chem. B, 110, 1787 (2006). 12